Devices, such as memory circuits, logic circuits and image sensors (e.g. CMOS sensors), are becoming more highly integrated these days. In a process for forming such a device, foreign matter, such as fine particles and dust, may sometimes adhere to the device. Foreign matter adhering to a device can cause short-circuit between interconnects or can cause a circuit defect. Therefore, in order to enhance the reliability of the device, it is necessary to clean a wafer having devices formed thereon so as to remove the foreign matter from the wafer. The above-described foreign matter, such as fine particles and dust, can adhere also to the back surface (non-device surface) of a wafer. If such foreign matter adheres to the back surface of the wafer, the wafer can separate from or become inclined with respect to a stage reference surface in an exposure apparatus, resulting in patterning deviation or deviation of focal distance.
In order to prevent such problems, it is necessary to remove foreign matter adhering to a back surface of a wafer. However, it has been difficult for a conventional cleaning technique, which involves scrub-cleaning a wafer with a pen-type brush or a roll sponge while rotating the wafer, to remove foreign matter covered with a film deposited thereon or to remove foreign matter from the entire back surface of the wafer. In order to address such problem, a technique has recently been proposed which can remove with high removal efficiency foreign matter not only from a bevel portion and a peripheral portion of the back surface of a substrate, such as a wafer, but from the entire back surface of the substrate.
This new technique involves rubbing a polishing tool against a back surface of a wafer to slightly scrape off the back surface, and can therefore remove foreign matter from the back surface with high removal efficiency. However, even when cleaning of the back surface of the wafer is performed by a conventional method after the polishing of the back surface, polishing debris may remain on the back surface of the wafer. After the wafer is dried, such polishing debris may leave the wafer in a wafer cassette and fall onto another wafer in the wafer cassette.